Electroplating aluminum alloys from organic solvent baths and articles coated therewith

ABSTRACT

A coating for metallic faces comprising an alloy of aluminum with at least one of zinc, cadmium or manganese is proposed, whereby the alloy coating is applied onto the metal surface by means of electrodeposition using a non-aqueous electrolyte. The electrolyte comprises toluene as a solvent for chlorides of the alloy components. The coating may be used e.g. for corrosion protection.

FIELD OF INVENTION

The invention relates to a coating for metallic substrates, the coatingcomprising an Al-alloy, as well as a method for producing this coating.The invention further relates to the use of this coating for protectingmetallic surfaces e.g. against corrosion.

BACKGROUND ART

It is known from U.S. Pat. Nos. 3,343,930 and 3,393,089 to coat metallicsurfaces with an alloy of zinc and aluminum for corrosion protection.The method according to which these alloys are coated onto the metallicsurfaces comprises dipping the metal substrate into a molten bath of therespective alloys. The advantages of zinc-aluminum coatings compared topure aluminum coatings or alloys of aluminum with e.g. refractory metalssuch as titanium, chromium or molybdenum is that zinc provides a highergalvanic protection to e.g. a steel substrate. Thus the steel surfaceswhich are coated with a zinc-aluminum alloy provide a better corrosionprotection than pure aluminum or alloys of aluminum with refractorymetals.

Even though the above two U.S. patents already disclose the use ofzinc-aluminum alloys for improved galvanic corrosion protection, theproposed hot dipping process does not provide optimum coatings inrespect of uniformity, optimal thickness and formability. In additionthe high temperature process leads to the formation of brittleintermetallic phases at the coating-substrate interface and requires theaddition of silicon to reduce this tendency and to improve the coatingadhesion.

The U.S. Pat. No. 4,287,009 discloses an Al/Zn coating which is appliedto the substrate by a hot dipping process, whereby the exact temperatureduring the dipping process and the cooling rate are controlled as toimprove the structure of the coating. Compared to uncontrolled dippingprocesses it is claimed to reduce the grain size and therewith improvethe performance of the coating. However, it is evident from the phasediagram of the Al/Zn system that no single phase solidification isobtainable since the area in the phase diagram which is designated as"α+L" has to be crossed during the cooling period, which needs a certainamount of time, during which the composition of the liquid phase changesgradually because of the solidification of selected components.Therefore the solidification leads first to Al rich zones and then to Znrich zones, thus creating at least a two phase coating. The grain sizesare coarse, being of the order of up to 50 microns. The only possibilityto obtain a single phase solidification in this system would be to passexactly through the eutectic point which is, however, only possible fora mixture of approximately 5% Al and 95% Zn. The shock cooling aftercomplete solidification may decrease the grain size thereafter byinterdiffusion, but does not lead to a real submicroscopic distribution,which is believed to yield the best performance for corrosionprotection, since relatively large Al-rich zones tend to becomepassivated by the formation of a protective oxide layer. Consequentlythe zinc is subject to corrosion leaving a porous passivated Alstructure which is less galvanically active and offers poorer corrosionprotection to the substrate.

From U.S. Pat. No. 3,775,260 it is known to electroplate aluminum ore.g. an alloy of aluminum and zinc or cadmium onto a metallic substratewhereby the electrodeposition is carried out in a hydrous organicsolvent comprising aluminum bromide. The second metal component of thedesired alloy is introduced as a soluble alloy anode which graduallydissolves as the respective metal component is plated onto the cathode.However, there are several disadvantages in the use of the hydrouselectrolyte. The reduction of protons will lead to a loss of cathodiccurrent efficiency. In addition the proton reduction can lead tohydrogen embrittlement problems when high strength steels are coated.Moreover, this U.S. patent does not refer to any use of the coatingsproduced with the disclosed metals and no specific advantages of theobtained coatings in respect to other coating methods are discussed.

U.S. Pat. No. 2,170,375 discloses a method for electroplating aluminumor an alloy thereof with zinc, copper or cadmium onto a substrate of adissimmilar metal in a non-aqueous bath comprising the reaction productof benzene or derivatives of benzene and alkyl or hydrogen halides.However, alkyl halides lead to the formation of alkylhalogenoaluminatespecies which are not cathodically stable. Consequently these speciescan be reduced during electrolysis leading to a decrease of currentefficiency and chemical decomposition of the solvent system. Again nospecific use of this method and no product which may be manufactured bythis method is disclosed and no specific advantages of this platingmethod over other known coating procedures are discussed.

H. G. Read et al (ELECTROCHEMICAL TECHNOLOGY, Vol 4, No. 11-12, Nov/Dec.1966) report electrolytic codeposition of aluminum and manganese infused-salt baths to form bright coatings. Details are discussed relatingto the optical aspects of these coatings depending on the concentrationof manganese in the electrolyte and in the deposit. The coatingsproduced from this relatively high temperature system are reported bythe authors to be crystalline.

There have been no reports of the direct electrochemical preparation ofamorphous aluminum alloys. In addition it has not proved possible toprepare amorphous aluminum manganese alloys by the conventionally usedrapid quenching technique.

OBJECT OF THE INVENTION

It is an object of the invention to improve the properties of protectivecoatings based on an alloy of e.g. group IIb and VIIb metals withaluminum, in particular to obtain a better surfaces quality, bettermicrostructure, better adhesion and thinner coatings which provide moreefficient use of the materials e.g. for galvanic corrosion protectionand improved formability and weldability.

It is another object of the invention to provide a method for producingthis coating which does not require high temperatures and is thereforeless expensive and which does not create problems due to the formationof brittle intermetallic phases.

It is another object of the invention to provide a plating system whichis stable during electrolysis and from which coatings can be platedwithout the risk of hydrogen embrittlement to the substrate beingcoated.

A further object of the invention is the provision of alloyingcomponents together with aluminum which may not be practicallyobtainable by the hot dipping process as is the case e.g. withaluminum-cadmium where the phase diagram shows no solubility between therespective solid and liquid states and in the case of aluminum-manganesewhere melt temperatures in excess of 850° C. are required to producecoatings with more than 10 w% manganese, and in excess of 1100° C. toproduce coatings with more than 50 w% manganese. Such high temperatureswill cause practical plant design problems and will be incompatible withmany substrate materials.

A still further object of the invention is to provide a method which mayunder certain conditions lead to the formation of thermally metastablecoatings in a non-equilibrium state, having e.g. amorphous structure.

SUMMARY OF INVENTION

The above objects of the invention are met by a protective coating asmentioned in the preamble characterized by the alloy components of thecoating being present in a homogeneous, submicroscopic distributionobtained by electroplating of the alloy in an organic electrolyte. In apreferred embodiment of the invention the aromatic hydrocarbon istoluene and the above referred halides of the alloy components arechlorides. The electrical conductivity of the above electrolyte may beimproved by addition of an alkali halide such as LiCl.

A coating according to the invention may comprise 5 to 95 w% of aluminumand 95 to 5 w% of zinc, cadmium or manganese.

The coating according to the invention has a finer microstructure thanthat produced by the hot dip process with grain sizes in thesubmicroscopic range, i.e. smaller than 1 micrometer. X-ray photos ofthe coating with magnification 4500 are shown in the drawings. Thissuper fine structure is due to the random deposition of zinc andaluminum atoms.

Being a further object of the invention to provide a method forproducing a corrosion protective coating as mentioned in the preamble, amethod according to the invention may comprise the steps of

(a) preparing an electrolyte comprising halides of the alloy componentsdissolved in an aromatic hydrocarbon; and

(b) electroplating a cathodically polarized metal surface in theelectrolyte according to "(a)"

The electrolyte as mentioned above may contain an alkali halide such asLiCl and the aromatic hydrocarbon may be toluene. The electrolyte usedto carry out the method according to the invention may comprise 0.1 to0.3 moles LiCl, 0.1 to 0.5 moles of AlCl₃ and one of 0.0003 to 0.003moles of ZnCl₂ or 0.001 to 0.005 moles of CdCl₂ or 0.005 to 0.05 molesof MnCl₂ all per mole of toluene.

The method according to the invention may lead to the formation ofcorrosion protective coatings comprising 5 to 95 w% of Al and 95 to 5 w%of zinc, cadmium or manganese.

Coatings produced according to this invention may have a variety ofdifferent structures. While it is possible to obtain pure crystallinestructures, it is also feasible to produce thermally metastablenon-equilibrium structures such as amorphous coatings depending on theprocess parameters and compositions of the electrolyte and the deposit.Amorphous materials as well as the recently reported quasicrystallinephase (Physics Today, February 1985, page 17) are thermally metastableand revert to the crystalline phase on heating to a certain criticaltemperature. The relatively low temperatures (around 60° C.) of theorganic electrolyte used to plate aluminum alloy coatings means that thethermally metastable phases which can be produced under the appropriateconditions of electrodeposition are stable under the conditions ofpreparation and do not convert to the more stable crystalline phase. Theability to produce amorphous coating structures is advantageous e.g. forapplications where properties such as corrosion resistance are required,since it is well known that amorphous materials are significantly moreresistant to corrosion than their crystalline equivalents. The use ofthese coatings in other areas, such as for chemical catalysts,decorative surface treatments, contact materials etc. may also beenvisaged.

Finally, the invention is therefore directed to the use of a coatingwhich is produced according to the above method for protecting metallicsurfaces against corrosion.

DETAILED DESCRIPTION OF INVENTION

Coatings consisting of an alloy of aluminum with zinc, cadmium ormanganese as a second component are produced by electroplating oncathodically polarized metal substrates in an electrolyte comprisingtoluene, AlCl₃ and a chloride of the mentioned second alloy component.With respect to the theory of electroplating in non-aqueous electrolytesvoltametric measurements with the respective electrolytes containingzinc, cadmium or manganese were carried out in order to establishoptimum operating parameters for the plating procedure and to determinethe influence of those parameters on the composition of the deposit. Asan example, one of these measurements was carried out in a solution of0.33 moles of AlCl₃, 0.215 moles of LiCl and 0.01 mole ZnCl₂ for 1,00mole of toluene. The voltametry showed a small cathodic wave at +160 mVvs the aluminum electrode which might correspond to the deposition ofpure zinc. This first wave was followed by a composite wave from -100 to-260 mV with a more or less defined plateau from -260 to -800 mV. On theanodic side, if the applied cathodic potential was more positive than 0mV, only one anodic peak was observed at +160 to +200 mV. If the appliedcathodic potential was between -100 and -800 mV a large dissolutionpeak, which was the superposition of three peaks at +50 +150 and +200 mVwas observed. The second alloy (+150 mV) was the predominant product inany case. The deposition of pure aluminum was observed only atpotentials more negative than -1000 mV. The composition of the depositwas also studied by galvanic electrolyses at a micro cathode ofplatinum. At a current density lower than 15 A/cm² the depositdissolution peak was observed from +100 mV.

At a current density between 15 and 40 mA/cm² the deposit dissolutionwas observed from +20 to +70 mV. At higher current densities a smallpeak of pure aluminum dissolution was observed at 0 mV.

At higher concentration at ZnCl₂ (6 mole %), the voltametry showed thatup to -600 mV only pure zinc or at least rich zinc alloy could beobtained.

As a result of these measurement it can be concluded that in spite ofthe lack of an intermetallic compound between zinc and aluminum a Zn--Alalloy having a defined dissolution potential could be deposited at adefined potential, from an AlCl₃ /aromatic hydrocarbon electrolyte.

The desired composition of the alloy deposit may be obtained by suitablechoice of the bath composition and the plating conditions as illustratedin TABLE 1.

According to the results of the microstructural analysis the mosthomogeneous coating structures can be obtained with a ZnAl compositionof 40 to 60 w% Zn.

According to these results the most useful composition of Zn--Al (lessthan or equal to 50 weight%) could be obtained with a bath containingabout 1 mole% of ZnCl₂.

The hardness of the zinc aluminum deposit (10-30 w% Zn) was about 50 to70 HV (Vickers Hardness) comparing to 40-50 HV for pure zinc, and about30 HV for pure aluminum. The free corrosion potential of zinc-aluminum(10 to 30 w% Zn) in NaCl 350 gpl, was -1090 mV vs SCE. This valueindicated that the galvanic protection power of Zn--Al should be betterthan for pure aluminum with a free corrosion potential of -650 to -950mV.

SAMPLE PREPARATION AND CHARACTERIZATION

Three types of substrates were used, comprising mild steel, stainlesssteel and high strength steel.

The following bath composition and experimental conditions were used forZn--Al (5/95 to 95/5) coatings:

LiCl: AlCl₃ =0.65, ZnCl₂ concentration: 1.0 mole % vs AlCl₃concentration, current density: 20 mA/cm². The samples were subjected totwo different kinds of test, the first one was the saline spray test andthe second were mechanical tests, which comprised the evaluation of themicrohardness (Vickers), the ductility and the adhesion.

EXAMPLES Example 1

In a glove-box, with a nitrogen atmosphere containing less than 10 ppmwater, 8 portions of LiCl--AlCl₃ --toluene solutions were prepared bymixing, at about 50° C., 0.215 moles of LiCl (9.125 g--Cerac 99.8%),0.330 moles of AlCl₃ (44.055 g--Fluka 99%), and 1.0 mole of toluene (92g--Merck "pro analysis").

Into one portion of solution, 0.020 moles of ZnCl₂ (2.728 g--Cerac99.5%) was added in order to obtain a solution containing about 6 mole %ZnCl₂ vs AlCl₃ (solution A).

Afterwards, 7 plating solutions, with different initial concentrationsof ZnCl₂, were prepared by adding a certain volume of solution A (4 to20 ml) to each of the 7 other portions of LiCl--AlCl₃ --toluenesolution.

The electrolyses were carried out in a cylindrical glass cell, at50°-60° C. The agitation was insured by a magnetic stirrer. Two Alanodes of dimension 2.5×6.0 cms were used. The cathode was a mild steelsubstrate of dimension 2.5.×6.0×0.1 cms. Before the deposition step, themild steel substrate was polarized anodically in order to obtain a cleanand activated surface. The respective anodic charges and currentdensities were in the range of 5-10 Asec/cm² and 5-10 mA/cm². After theanodisation step, the deposition of Zn--Al alloys was effected byinversion of the electrode polarity.

In order to avoid a strong modification of ZnCl₂ concentration insolution, the electrolyses are stopped after passage of a cathodiccharge of 15 Asec/cm². A grey deposit of about 5-7 micrometers ofthickness was obtained on the immersed surface of the substrate. Thedeposits were dissolved in 10% HCl and the composition was analyzed byatomic absorption.

The results listed in Table 2 show the influences of the initialconcentration of ZnCl₂ in solution, and the current density on thedeposit composition.

                  TABLE 1                                                         ______________________________________                                        ZnCl.sub.2 in solution                                                                    i.sub.c    Deposit composition (w %)                              (mole % vs AlCl.sub.3)                                                                    (mA/cm.sup.2)                                                                            Al         Zn                                          ______________________________________                                        0.84        20         34         66                                          0.80        20         42         58                                          0.84        30         56         44                                          0.77        30         56         44                                          0.70        30         56         44                                          0.31        30         66         34                                          0.17        30         84         16                                          ______________________________________                                    

The coating adherence was tested by bending tests, no cracks nor peelingof the coatings were observed. The ductility of the coatings wasestimated following the ASTM-B489 standard, all the coatings withcompositions listed in Table 1 showed experimental elongations of 100%or higher.

The microhardness (Vickers) of the coatings, which increases with thecontents of Zn, was in the range of 44 to 50 HV.

Example 2

Three solutions of ZnCl₂, LiCl, AlCl₃ and toluene, with ZnCl₂concentrations of 1.0, 0.5 and 0.3 mole % versus AlCl₃ concentrationwere prepared as in example 1.

The electrolyses were carried out under similar conditions as describedin example 1. The current density was in the range of 20 to 30 mA/cm²,and were chosen following the initial concentration of ZnCl₂ in order toobtain ZnAl deposits containing about 30 to 80% of Zn. Four samples ofZnAl coated mild steel of 8-10 micrometers thickness, with differentdeposit compositions were obtained.

The metallographic samples of these coatings were prepared and themicrostructure of the deposits was examined by making the mappinganalysis of Zn and of Al by the SEM method.

Example 3

Four solutions of ZnCl₂, LiCl, AlCl₃ and toluene were prepared as inexample 1.

The electrolyses were carried out under the same conditions as describedin example 1. Two Al anodes, and two new Zn anodes were used for eachelectrolysis. Two rectifiers were used in opposition in order to adjustthe partial currents through the Al and Zn anodes. The cathodic chargeswere calculated in order to obtain a quantity of Zn deposited, under theform of ZnAl alloys, corresponding to about 50% of the total quantity ofZn in solution. The mass balance of ZnCl₂ in solution is determined bythe weight loss of Zn anodes, by the composition analyses of thedeposits and the electrolyte before and after electrolysis.

The results listed in Table 2 show the influence of the anodic currentdensity and the current ratio between Al and Zn anodes (i(Zn):i(Al)) onthe current efficiency of the Zn dissolution and the ZnCl₂ mass balance.

                  TABLE 2                                                         ______________________________________                                                                                ΔZn                             i.sub.a i(Zn)   Init. Zn Deposit CE diss.                                                                             (sol.)                                (mA/cm.sup.2)                                                                         i(Al)   (mole %) Zn (w %)                                                                              Zn(%)  (%)                                   ______________________________________                                        20      0.25    0.98     66      118    -22.4                                 30      0.25    0.84     44      111    -17.9                                 30      0.25    0.84     44       96    -26.2                                 20      0.43    0.80     58      103    0                                     ______________________________________                                         Note:                                                                         (1) The current efficiency of the anodic dissolution of Zn higher than        100% may be due to the reaction: Zn + 2H.sup.+ → Zn.sup.2+  +          H.sub.2 -                                                                     (2) ΔZn (solution) = (Zn.sub.initial + Zn.sub.dissolved) -              (Zn.sub.deposited + Zn.sub.final)                                             ΔZn is calculated relative to the initial concentration of              ZnCl.sub.2 in solution.                                                       The negative value of ΔZn is due to the concentration of Zn.sup.2+      onto the back of the Al anodes following the reactions: 2Zn.sup.2+  + 2Al     → 2Al.sup.3+  + 3Zn.                                              

Example 4

A solution CdCl₂ : LiCl; AlCl₃ : toluene of composition (in mole units)0.003: 0.215: 0.330: 1.0 was prepared by mixing 0.550 g CdCl₂(Cerac--99.5%), 9.125 g LiCl (Cerac--99.8%), 44.055 g AlCl₃ (Fluka--99%)and 92 g toluene (Merck--"pro analysis").

The electrolysis was carried out in a cylindrical glass cell asdescribed in example 1. Two Al anodes were used. The cathode was a mildsteel substrate of dimensions 2.5×6.0×0.1 cms. The anodisation of thesubstrate was carried out at 7.5 mA/cm² with a charge of 5 Asec/cm². Thesubsequent deposition was effected at 20 mA/cm². After the passage of acathodic charge corresponding to 35 Asec/cm², a "grey-silver" deposit ofabout 20 micrometer thickness was obtained on the immersed surface ofthe substrate. One part of the coating was dissolved in 10% HCl, and thequalitative analysis of the resulting solution showed the presence of Cdand Al.

The coating adherence onto mild steel substrate was tested and proved bythe bending test. The coating ductility, estimated following theASTM-B489 standard, is about 100% of elongation. The micro hardness(Vickers) is about 54 HV. One sample of CdAl coated mild steel ofdimensions 2.5×5.0×0.1 cms was heat treated in air, successively at400°, 500° and 600° C. during 1 hour at each temperature, and nodestruction of the coating by Cd fusion was observed, only a surfaceoxidation of Al.

Example 5

A solution MnCl₂ : LiCl; AlCl₃ : toluene of the composition (mole unit)0.010: 0.215: 0.330: 1.0 was prepared by mixing 1.258 g MnCl₂(Cerac--99.8%), 9.125 g LiCl (Cerac--99.8%), 44.055 g AlCl₃ (Fluka--99%)and 92 g toluene (Merck--"pro analysis").

The electrolysis was carried out in a glass cell as described inexample 1. Two Al anodes were used. The cathode was a mild steelsubstrate of dimensions 2.5×6.0×0.1 cms. The anodisation of thesubstrate was effected at 10 mA/cm² with a charge of 5 Asec/cm². Afterthe passage of a cathodic charge corresponding to 35 Asec/cm², a darkgrey deposit of about 10 micrometers was obtained on the immersedsurface of the substrate. The qualitative analysis of the coating,dissolved in HCl 10%, showed the presence of Mn and Al.

The coating adherence was tested and proved by the bending tests. Thecoating ductility estimated following the ASTM-B489 standard, wasbetween 30 and 50% of elongation. The micro hardness (Vickers) of thecoating was about 250 HV.

Example 6

Four samples of Zn--Al, one sample of Cd--Al and one sample of Mn--Alcoated mild steel (dimensions 2.5×6.0×0.1 cms) were prepared followingthe conditions described in examples 1, 4 and 5. The mild steelsubstrate in all cases were cleaned and activated by anodisation in thesame plating bath, at 5 mA/cm², with an anodic charge corresponding to 5Asec/cm². Afterwards, the samples were submitted to the saline spraytest, without any post-treatment. The results are listed in Table 3.

                  TABLE 3                                                         ______________________________________                                        Characteristics   Performances                                                              Thick   Initial                                                                             Test                                                    Comp.   ness    Potent.                                                                             Time  Pot.                                        Type  (w %)   (μm) (mV)* (hrs.)                                                                              (mV)*  Comments                             ______________________________________                                        Zn/Al 18-82   10      -1170  510  -340   (A) red                                                                       rust at                                                                       420 hrs                              Zn/Al 35-65   10      -1140 1040  -450   (A) red                                                                       rust at                                                                       635 hrs                              Zn/Al 45-55   10      -1150 1040  -370   (A) red                                                                       rust at                                                                       635 hrs                              Zn/Al 60-40   10      -1150 1135  -1040  (A) no red                                                                    rust                                 Cd/Al         10       -840  385  -830   (B) no red                                                                    rust                                 Mn/Al         10       -895  580  -620   (B) no red                                                                    rust                                                             1444  -629   no red                                                                        rust                                                             2668  -578   no red                                                                        rust                                                             2860  -540   60% red                                                                       rust                                 ______________________________________                                         *Saturated NaCl, SCE Reference Electrode                                      (A) Edges exposed  (B) Edges covered, coating scribed.                   

Example 7

Four samples of ZnAl coated mild steel (dimensions 2.5×6.0×0.1 cms) wereprepared following the conditions described in example 1. The mild steelsubstrate in all cases were sandblasted, cleaned in water, activated inHCl 30% for 5 minutes, and rinsed in water, and acetone. Afterwards, thedried substrates were introduced into the glove-box. The depositionswere made immediately, without any anodisation. The adherence of thecoating was successfully tested by bending tests.

The samples were submitted to the saline spray test as described inexample 6. The results are listed on the Table 4.

                  TABLE 4                                                         ______________________________________                                        Characteristics    Performances                                                             Thick    Initial                                                                             Test                                                   Comp.   ness     Potent.                                                                             Time  Pot.                                       Type  (w %)   (μm)  (mV)* (hrs.)                                                                              (mV)* Comments                             ______________________________________                                        ZnAl  10-90   10       -1085  72   (B) no red rust                            ZnAl  30-70    5       -1150 192   (B) no red rust                            ZnAl  40-60    5       -1150 192   red rust at 72 hrs.                        ZnAl  50-50   10       -1120 192   red rust at 92 hrs.                        ______________________________________                                         *saturated NaCl  SCE Reference electrode                                      (B) Edges covered, coating scribed.                                      

Example 8

A solution of MnCl₂, LiCl and AlCl₃ in toluene was prepared from 10 gMnCl₂ (Cerac--99.8%), 30.6 g LiCl (Cerac--99.8%), 160 g AlCl₃(Fluka--99%) and 330 g toluene (Merck--"pro analysis").

The electrolysis was carried out in a glass cell as described inexample 1. After passage of a cathodic charge of 750 Asec a deposit ofabout 4-5 micrometers thickness was obtained on the steel substrate.Elemental analysis of the coating in a Scanning Electron Microscopeshowed it to contain about 20 w% Mn and 80 w% Al.

A section of the coating was removed from the steel substrate andinvestigated by Transmission Electron Microscopy. The transmissionelectron diffraction pattern obtained showed the coating to beamorphous. On heating the material the electron diffraction patternchanged and a spot pattern was obtained characteristic of thecrystalline phase Al₆ Mn. This confirms the metastable nature of thecoating as deposited.

Example 9

A solution of MnCl₂, LiCl and AlCl₃ in toluene was prepared from 16.8 gMnCl₂ (Cerac--99.8%), 33.9 g LiCl (Cerac--99.8%), 177.8 g AlCl₃(Fluka--99%) and 368.6 g toluene (Merck--"pro analysis").

The electrolysis was carried out in a glass cell as described inexample 1. After passage of a cathodic charge of 1300 Asec at a currentdensity of 20 mA/cm₂ a deposit of about 22 micrometers thickness wasobtained on the steel substrate. A section was cut from the sample andthe deposit was dissolved in 20% HCl. The resulting solution wasanalyzed by atomic absorption spectroscopy. The results obtained showedthat the coating contained about 35 w% Mn and 65 w% Al.

The remaining piece of the coated sample was subjected to X-raydiffraction analysis. The X-ray diffraction pattern obtained with CuK_(alpha) radiation consisted of a broad peak spread over the 2 thetarange of 35°-50° centered at 42.6°. On heating the sample for fiveminutes at 400° C. in argon the broad peak in the X-ray diffractionpattern was resolved into sharp diffraction lines, illustrating thethermally metastable nature of the deposited coating.

What is claimed is:
 1. A metallic substrate coated with a coating of analuminum-manganese alloy, characterized in that the coating is ahomogeneous, amorphous alloy of aluminum-manganese containing 95 to 5 w%manganese and a balance of aluminum, and having a grain size smallerthan 1 micrometer, said homogeneous, amorphous aluminum-manganese alloycoating having been obtained by electroplating onto the substrate froman essentially water-free electrolyte comprising 0.1-0.3 mole of analkali halide, 0.1 to 0.5 mole of an aluminum halide and 0.005-0.05 moleof a manganese halide per mole of an aromatic hydrocarbon solvent, saidhomogeneous amorphous aluminum-manganese alloy coating having athermally metastable state which is stable under the conditions ofpreparation but can be converted to a crystalline state by heating to acritical temperature.
 2. The coated substrate of claim 1, wherein thehomogenous, amorphous alloy coating contains from about 20 w% Mn 80 w%Al to about 35 w% Mn 65 w% Al.
 3. A method of electroplating a metallicsubstrate with a coating of an aluminum-manganese alloy, comprising thesteps of:(a) preparing an essentially water-free electrolyte comprising0.1-0.3 mole of an alkali halide, 0.1 to 0.5 mole of an aluminum halideand 0.005 to 0.5 mole of a manganese halide per mole of an aromatichydrocarbon solvent and (b) cathodically polarizing the metallicsubstrate in the electrolyte to deposit a coating of a homogeneous,amorphous alloy of aluminum-manganese containing 95 to 5 w% manganeseand a balance of aluminum, and having a grain size smaller than 1micrometer, said homogeneous, amorphous aluminum-manganese alloy coatinghaving a thermally metastable state which is stable under the conditionsof preparation but can be converted to a crystalline state by heating toa critical temperature.
 4. The method of claim 3, wherein saidelectrolyte preparation provides 0.1-0.3 mole of LiCl, 0.1 to 0.5 moleof AlCl₃ and 0.005 to 0.5 mole of MnCl₂ per mole of toluene.
 5. Themethod of claim 4, wherein the resulting homogeneous, amorphous alloycontains from about 20 w% Mn 80 w% Al to about 35 w% Mn 65 w% Al.